lit is required to measure the NOX (nitrogen oxides) emissions produced by combustion plant to ensure that environmental standards are met. The two most important constituents of NOX are NO (nitric oxide) and NO2 (nitrogen dioxide). NO is usually the predominant specie but NO2 is far more toxic and reactive. Currently to measure accurately NOX at low levels extractive gas analysis must be used. A probe extracts a sample of the emissions, and a transfer line conveys the sample to measurement instrumentation which measures the NO and NO2 content. It has been found that the materials of which the probe and transfer line are made may alter the emission sample so that the measurement instrumentation does not give an accurate measure of NO/NO2 content. A metallic probe may convert NO2 to NO resulting in an artificially low measure of NO2. This especially occurs at high temperatures, i.e. temperatures of 500 degrees Celsius and above as found for example in a gas turbine engine exhaust. The material of the transfer line may absorb NO2, for example a transfer line made of polytetrafluoroethylene (PTFE). In contrast to NO2, NO is very stable. In conclusion, current approaches to measuring NOX tend to under-measure total NOX and especially NO2.